Let me help you....I have shot the 18". 20" and the 24" and the only difference I can tell is the weight. The 24" Varmint rifle is HEAVY!!! I mean more than I want to pack around. By the time you add a scope, mount and a bipod, you are knocking on 14LBS.
I have the 18" and it shoots great. My next Varmint upper is going to be a 16"....
IMO, the 20" is a good compromise on a good all around shooting Varmint upper.